I. Field of the Invention
The present invention relates to digital wireless communication systems. More particularly, the present invention relates to a novel and improved method for scanning other-frequency pilot signals in a code-division multiple access (CDMA) communication system.
II. Description of the Related Art
In the field of wireless communications, several technology-based standards exist for controlling communications between a mobile station, such as a cellular telephone, Personal Communication System (PCS) handset, or other remote subscriber communication device, and a wireless base station. These include both digital-based and analog-based standards. For example, among the digital-based cellular standards are the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) Interim Standard IS-95 series including IS-95A and IS-95B, entitled "Mobile Station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System." Similarly, among the digital-based PCS standards are the American National Standards Institute (ANSI) J-STD-008 series, entitled "Personal Station--Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems." Other non-CDMA based digital standards include the time-division multiple access (TDMA) based Global System for Mobile Communications (GSM), and the U.S. TDMA standard TIA/EIA IS-54 series.
The spread spectrum modulation technique of CDMA has significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31, 1992, entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the present invention and incorporated by reference herein.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", Ser. No. 07/433,031, filed Nov. 7, 1989, also assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
The aforementioned patents all describe the use of a pilot signal used for acquisition in a CDMA wireless communication system. At various times when a wireless communication device such as a cellular or PCS telephone is energized, it undertakes an acquisition procedure which includes, among other things, searching for and acquiring the pilot channel signal from a base station in the wireless communication system. For example, demodulation and acquisition of a pilot channel in a CDMA system is described in more detail in copending U.S. patent application Ser. No. 08/509,721, entitled "METHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM," assigned to the assignee of the present invention and incorporated herein by reference. When more than one pilot channel can be acquired by the wireless communication device, it selects the pilot channel with the strongest signal. Upon acquisition of the pilot channel, the wireless communication device is rendered capable of acquiring additional channels from the base station that are required for communication. The structure and function of these other channels is described in more detail in the above referenced U.S. Pat. No. 5,103,459 and will not be discussed in detail herein.
The above standards and patents describe, among other things, the manner in which a mobile station is to execute a "handoff" between neighboring base stations as it travels between their respective geographic coverage areas. For example, in the CDMA-based standards IS-95 and J-STD-008, the base station sends a message to the mobile station listing many of the system parameters of its neighboring base stations, including such information as would assist the mobile station in executing an "autonomous" handoff between base stations. An autonomous handoff is one that is not initiated or directed by the base station, but rather is initiated by the mobile station itself.
An example of one such neighbor list message is the "Extended Neighbor List Message" of J-STD-008. When the base station sends an Extended Neighbor List Message to the mobile station, it uses the format of Table I.
TABLE I ______________________________________ Field Length (bits) ______________________________________ MSG.sub.-- TYPE (`00001110`) 8 PILOT.sub.-- PN 9 CONFIG.sub.-- MSG.sub.-- SEQ 6 PILOT.sub.-- INC 4 Zero or more occurrences of the following record: NGHBR.sub.-- CONFIG 3 NGHBR.sub.-- PN 9 SEARCH.sub.-- PRIORITY 2 FREQ.sub.-- INCL 1 NGHBR.sub.-- BAND 0 or 5 NGHBR.sub.-- FREQ 0 or 11 RESERVED 0-7 (as needed) ______________________________________
The above table is taken from Section 3.7.2.3.2.14 of J-STD-008, and indicates the various fields transmitted in an exemplary Extended Neighbor List Message. Of particular concern to the present invention are the following fields:
NGHBR.sub.-- PN--the base station sets this field to the pilot PN sequence offset for this neighbor, in units of 64 PN chips; and
NGHBR.sub.-- FREQ--the base station sets this field to the CDMA channel number corresponding to the CDMA frequency assignment for the CDMA channel containing the paging channel that the mobile station is to search.
Thus, according to J-STD-008, the mobile station is given the frequency and PN offset of each neighboring base station. This gives the mobile station enough information to make a more focused search for neighbor pilots, rather than having to search all possible PN offsets on all possible CDMA frequency assignments. For example, the mobile station may keep a table of all the neighbors that were passed to it in the neighbor list message or extended neighbor list message. Such a table might resemble Table II below.
TABLE II ______________________________________ PN Offset (chips) Frequency ______________________________________ 12 f(1) 24 f(1) 48 f(1) 12 f(2) ______________________________________
With respect to "same-frequency neighbors," i.e. those that are on frequency f(1), the very nature of a CDMA modulation scheme allows a mobile station with a diversity receiver such as that described in the above-mentioned U.S. Pat. No. 5,109,390 to search for other pilot signals on the same frequency assignment, but having different PN offsets, while simultaneously continuing to demodulate any channel that it is already monitoring. In other words, a CDMA mobile station is typically able to search for the pilot signals of other base stations on the same frequency assignment, without interrupting transmission or reception of data with its original base station.
However, a significant problem remains with regard to acquiring "other-frequency neighbors," i.e. those that are on frequency f(2). Namely, if a neighboring base station's pilot is on a different frequency assignment, the mobile station is required to re-tune its synthesizer to that other frequency in order to receive and demodulate that other-frequency neighbor's pilot channel. During the time that it takes to re-tune, acquire, and determine whether that other-frequency neighbor is a good candidate for handoff, the mobile station is unable to either receive or transmit on its original frequency assignment. Clearly, this would cause undesirable problems such as missed information on the original frequency.
For example, suppose the mobile station has re-tuned its synthesizer to search for a particular other-frequency neighbor's pilot signal during the time that an incoming page addressed to the mobile station is being transmitted by the original base station. In such a case, the mobile station would miss the page, and therefore could not complete an incoming call. Furthermore, this would be undetectable by the mobile station's user, who would never be aware that he had missed an incoming call because the mobile station had temporarily re-tuned to the other frequency.
What is needed is a method and apparatus for scanning other-frequency neighbors which avoids missing incoming messages from the original base station.